US ornithologists have begun working with the American space agency Nasa to study bird migration in greater detail.

The Nature Conservancy is paying for the use of one of the agency's hi-tech radars, which allows it to follow the movements of neotropical songbirds.

The radar, sited on the western shore of Chesapeake Bay in Virginia, is one of only two in the whole of the US.

The conservancy says the work will help it to identify vital feeding areas on the birds' southerly migration routes.

Weather studies

Barry Truitt of the conservancy said radar ornithology began almost 60 years ago, but took off in the US only with the use of "nexrad" systems - next generation weather surveillance radars.

You have to have someone out there on the ground who can tell you these really are birds, and not insects for instance - dragonflies show up on the radar just like birds

Barry Truitt, Nature Conservancy

The system Nasa is renting to the conservancy is a polymetric radar, used normally to study rainfall.

Barry Truitt told BBC News Online: "They were explaining to me the physics of a falling raindrop, and how this radar let them look at it horizontally, vertically, from above and from below, and even determine the size of it.

"So I asked whether they could see birds with it. And they said: 'Oh sure, but we turn on all these filters to get rid of the birds'. And I said: 'Well, can you turn the filters off?' They said: 'Yeah, no problem'.

"Now Nasa collects rainfall data when it's raining, and I get access to the radar when it's not. This is really pioneering work."

High mortality

The neotropical songbirds hatch in Canada and the north-eastern US and winter in the Caribbean, central and South America. They use the area around Chesapeake Bay as a staging post, and the radar is revealing the flightpaths and the feeding sites they use.

Mr Truitt said: "Work's been done on where they breed and where they winter, but this is the first study on a landscape scale of an area they migrate through.

"Apart from something like 10-12 million short-distance migrants, it's estimated 5-6 million neotropical birds go through here, and 80% of the young of each year die on their first migration.

"If the habitat isn't there for the survivors to replenish their fat reserves, they won't have the energy to go any further."

Unidentifiable flying objects

Barry Truitt said the information the conservancy is gaining from using Nasa's radar would enable it to help the migrants.

He said: "It will help us to identify what habitats they're using, and to prioritise how we spend our money, for instance what kinds of trees we should be protecting.

"But there's still a problem with calibrating the data. Radar data is no good without verification.

"You have to have someone out there on the ground who can tell you these really are birds, and not insects for instance - dragonflies show up on the radar just like birds."